Positive displacement fluid measurement systems can be used to measure a fluid flow rate or volume. For example, dispensing systems can use feedback from a positive displacement fluid meter to control the volume of fluid dispensed. Such control systems can be used in lieu of time-on controls to more accurately dispense precise amounts of fluid.
One type of positive displacement fluid measurement system is a gear flow meter, e.g. an oval gear or lobe meter. A traditional oval gear meter provides a pair of oval gears positioned within an oval gear chamber such that the gears rotate in concert. A lobe meter, provides a pair of lobe elements within a chamber which interlock and rotate about respective axes. In each case, fluid enters the chamber through a fluid inlet and causes the gears to rotate, allowing fluid to pass around the gears to a fluid outlet within precisely measured pockets. In an oval gear meter, the pockets are defined between the rotating oval gears and the inner chamber wall. In a lobe meter, spaces between the lobes of provide the pockets. Ideally, in each case, none of the fluid being metered passes directly between the gears themselves, so that the volume of fluid exiting the chamber during each rotation is known. Thus the volume of fluid flow through a gear meter can be measured by measuring the number of rotations of the gears. Likewise flow rate can be determined from the speed with which the gears rotate.
For measuring the gear rotation, gear meters frequently include additional gears. For example, an oval gear meters can include timing gear systems located external of the chamber to translate the number of rotations of the oval gears into an appropriate signal. Oval gear and other positive displacement flow meters utilizing timing gear systems have the disadvantage that the cover of the gear chamber must include one or more shaft apertures for the shafts coupling the gears to the external timing gears. Fluid leakage can occur through these shaft apertures which can decrease precision of the measurement and waste fluid product.
Commonly owned U.S. Patent Application Publication No. 2008/0202255 A1 describes an oval gear meter incorporating a magnetic sensor for measuring the rotation of the oval gears with reference to a permanent magnet installed within at least one of the oval gears. Such a system addresses the issue of openings or apertures within the oval gear chamber by providing a magnetic detection of rotation through a substantially sealed chamber. However, it is often the case that the magnets must be sealed from the fluid, for example, when the meter is used with aggressive liquids. Complicated mechanics are required to seal the magnets within the gears which render such gears expensive and difficult to implement on a small scale.
Moreover, as the meter resolution is increased, the strength, size, and use of magnetic gear meters become limited. For very small meter resolutions (e.g. less than 0.5 ml), the use of magnetic gear meters can be precluded due to magnetic field interference, magnet size, and sensor location.